ES2574603T3 - Element with variable stiffness controlled by negative pressure - Google Patents

Abstract

Element with variable stiffness controlled by negative pressure. Element (1) with variable stiffness controlled by negative pressure, the element comprising: - a tightly sealed envelope (10); - a plurality of flexible layers (30, 30a, 30b, 30c, 30d, 30e) in the envelope, each layer (30, 30a, 30b, 30c 30d, 30e) having a first surface (31, 41) and a second surface (32, 42); and, - a valve (20) adapted to evacuate the inside of the casing (10); characterized in that: - the first and second surfaces (31, 41, 32, 42) of two adjacent layers have a coefficient of friction with each other that is greater than 0.5; - the first and second surfaces (31, 41, 32, 42) of two adjacent layers have adhesion properties such that a normal force per unit area below 0.07 N / mm is necessary

Description

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DESCRIPTION

Element with variable stiffness controlled by negative pressure Technical field

The invention relates to an element with variable stiffness controlled through negative pressure, such as aspiration or ford. The present invention is applicable:

- to certain medical devices (such as casts, plasters, functional orthotics, templates and emergency medical devices, such as splints for limbs and first aid elements for the whole body),

- to sporting goods (such as skates, ski boots, surfboards and protective equipment for sports, such as helmets or knee pads and pectoral protectors),

- to safety elements that harden in case of a crash or accident,

- for construction elements, for example, to be used for the manufacture of reconfigurable camping furniture or toys,

- to mold elements for the production of composite materials,

- to packaging elements, or

- to safety elements that harden in the event of a crash or accident, for example, in the automotive field.

State of the art

The use of negative pressure, such as aspiration or vacuum, is known to provide a way of converting an element from a flexible state, in which the element is formed and can easily be adapted to fit a specific desired shape (such as a part of the human body), to a rigid state, in which the element is rigid and provides support, protection and / or stabilization, and vice versa. The basic structure of devices that use negative pressure generally comprises internal charges that are normally mobile particles and a thin, flexible, air-impermeable outer shell. The structure normally allows the device to be installed easily and quickly around the body and the affected limbs. When the device is returned as desired in the desired position, it is subjected to negative pressure and the atmospheric pressure then compresses the flexible outer shell and applies substantial pressure to the entire mass of particles. The frictional force between the particles and the cover resists relative movement between them, thus providing stiffness. Usually, a valve is included to tightly close the cover when it is evacuated to maintain the rigidity of the device. From the rigid state the flexible state is normally obtained by opening the valve and blowing.

Several patents on orthopedic devices that employ negative pressure have been published. US Patent 2005/0137513 discloses a structure to maintain a homogeneous thickness in the devices for supporting and stabilizing a person or body parts with injuries. The device has an inner region wrapped by two flexible films and the inner region is divided into two insertion bodies that are formed, respectively, with two strips of flexible, air permeable material. Each insertion body is divided into chambers that contain loose particles, by way of intersecting seams formed between the strips of material. The seams in both insertion bodies are staggered in both directions, so that the particles combine to form a substantially homogeneous thick layer of particles. However, such a structure composed of granules or particles has the problem of being too thick, which leads to practical limitations, such as transport size, and a high volume that leads to problems such as a long evacuation time. to reach the desired level of negative pressure.

In order to solve the problem of unwanted thickness and volume, the body tightness element with controlled stiffness described in WO 2011/07985 is made of a gas-tight envelope that encloses a plurality of layers and has a valve adapted to evacuate the inside of the envelope. Each layer is made of a core made of a material with a high Young's modulus and flexibility, coated on both sides with a coating or coating layer made of a material with a high coefficient of friction. However, this type of body adjustment element presents the following problems:

- Once a vacuum has been applied and the body adjustment element is in its rigid state, when the valve is opened to move to the flexible state, the layers can remain attached to each other due to their stickiness, and therefore the state Flexible does not recover properly.

- In the rigid state, when under a flexural tension, the layers can be delaminated (disconnecting the core lining).

- The body adjustment element described in WO 2011/07985 includes some strips of a material with a low coefficient of friction, in order to correctly recover the flexible state at atmospheric pressure.

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However, these strips reduce the effective friction surfaces, which consequently reduces the stiffness of the element in a rigid state and therefore affects its correct operation.

In summary, an element with controllable stiffness is required, which efficiently resolves both stickiness and delamination problems, in the scenario described above.

Description of the invention

The present invention relates to a variable stiffness element controlled by negative pressure according to claim 1. Preferred embodiments of the element are defined in the dependent claims.

It is an objective of the present invention to provide an element of variable stiffness controllable by negative pressure that overcomes the problems of stickiness and delamination that existing controllable stiffness elements have. In order to do so, the element with variable stiffness controllable by negative pressure of the present invention is completely reversible between the flexible and rigid states, while maintaining or even improving, the stiffness relationship between the rigid state and the flexible compared to this relationship of rigidity of current solutions.

The element of variable stiffness controlled by negative pressure of the present invention comprises:

- a gas impermeable wrap;

- a plurality of flexible layers within the envelope, each layer having a first surface and a second surface; Y,

- a valve adapted to evacuate the inside of the envelope.

According to a first aspect of the invention, the first and second surfaces of two adjacent layers:

- have a coefficient of friction between them that is greater than 0.5; Y

- have certain adhesion properties, such that a normal force per unit area of not more than 0.07 N / mm2 is required to separate them and / or the energy per unit area required to separate them in the normal direction is below of 6.7 J / m2.

Therefore, the element of the present invention has a laminar structure comprising several layers and having the following properties:

- high coefficient of friction between the layers due to the selection of the materials used and the fact that the entire surface of the layers supports friction; Y

- Low adhesion between layers, especially when there is no normal force.

In this way the first and second surfaces can slide relative to each other at atmospheric pressure. An additional advantage of the present invention is that the layers may be thinner than the layers of prior art elements (which included layers with a core and a coating), due to the fact that the layers may be made of a single material or composite material that has the necessary properties of high friction coefficient and low adhesion, thus eliminating the need for both coating and core, and the corresponding adhesive layer between them.

With this specific configuration of the element of the invention, the relation of rigidity of the element between its flexible and rigid states is improved:

- first, because the layers do not stick together at atmospheric pressure, which makes the element more flexible and easier to form in its flexible state; Y

- Secondly, since there is no need for low friction strips or seams, the friction surface is increased which makes the element more rigid in its rigid state.

Depending on the applications of the element, the layers may be made of a single material, or they may be made of a matrix reinforced with a plurality of fibers. In both cases, the layers can be made of a continuous or homogeneous sheet, or the layer can be made of a woven structure of strips or ribbons.

When the layers are made of a matrix reinforced with fibers, the fibers can be unidirectional, bidirectional or multidirectional. Only composite materials that have unidirectional fibers are suitable for the structure woven with ribbons.

The fibers are preferably selected from glass, carbon, aramid or polyester fibers. And the matrix is preferably manufactured from a thermoset polymer or a thermoplastic polymer.

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According to another preferred embodiment, the layers may comprise a core coated by at least one coating on one side of the core. Preferably the core is coated by respective first and second coatings, a coating on each side of the core.

Such coatings are preferably made of a thermoplastic polyurethane elastomer.

The adhesion properties of the layers are preferably measured by an adapted version of a standardized method, such as ASTM D2979-01, "Standardized test method for pressure sensitive stickiness of adhesives using an inverted probe machine" ("Standard test method for pressure-sensitive tack of adhesives using an inverted probe machine "). Preferably, the adhesion properties are measured using a probe tack test with a preload equivalent to atmospheric pressure and a preload time of 100s.

The friction coefficient is preferably measured using a standardized procedure such as ASTM D 1894 "Standardized test method for static and kinetic friction coefficients of film and plastic sheets" ('Standard test method for static and kinetic coefficients of friction of plastic film and sheeting ').

In the case of layers made of woven tapes, the adhesion and friction properties can be measured using the adapted probe tack test on a specific number of tapes placed adjacent to each other on a flat surface, such that The resulting width is greater than the minimum value set by the standard.

The adhesion properties of the layers are measured on layers as they are in their conditions of use, that is, although the standard indicates that the layers must be cleaned before any measurement is made, to measure the adhesion properties of the layers. Element layers in the present invention it is not necessary to clean the layers previously; its adhesion and friction properties must be measured in the same conditions as when they are in use inside the element.

Other advantages and features of the invention will be apparent from the following detailed description and are particularly noted in the appended claims.

Brief description of the drawings

To complete the description and in order to provide a better understanding of the invention, a set of drawings is provided. Such drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be construed as restrictive of the scope of the invention, but only as an example of how the invention can be carried out. The drawings comprise the following figures:

Figure 1 shows a sectional view of an element of variable stiffness according to the invention.

Figure 2 is a perspective view of a first embodiment of the layers inside the element.

Figures 3, 4, 5, 6 and 7 show perspective views of other possible embodiments of the layers inside the element.

Figure 8 is a perspective view of a section of an element according to the invention intended for medical applications.

Figure 9 is a top view of an embodiment structure of the element.

Figure 10 schematically shows the method of adhesion of the layers.

Description of a preferred embodiment

The following description should not be taken in a limiting sense but is provided only for the purpose of describing the general principles of the invention. The following embodiments of the invention will be described by way of example, with reference to the aforementioned drawings showing the elements and the results according to the invention.

Figure 1 shows a preferred embodiment of an element 1 with variable stiffness according to the invention. The element 1 comprises a stretch wrap 10 that encloses a plurality of flexible layers 30 and a valve 20 adapted to evacuate the inside of the wrap. The gas-tight envelope 10 is suitable to be subjected to a controlled pressure and has a valve 20 adapted to evacuate the interior of the envelope.

In a known manner, when the interior of the envelope 10 is at atmospheric pressure, the layers 30 are decompressed. When vacuum is applied, the flexible layers 30 compress each other increasing friction between them, which in turn increases the stiffness of the element 1 as a whole. In this way, element 1 has variable state possibilities, from a soft state, flexible to atmospheric pressure to a rigid state when depressurized.

of fabric of ribbons of the layers according to another adapted used in the present case to measure the

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The novelty of the present element 1 resides in the specific structure and in the material of the layers 30, as shown in Figure 2; Each layer 30 is made of a single material or composite material with a first surface 31 on one side and a second surface 32 on the other side.

In order for element 1 to function properly and for its flexible or soft state to recover properly once the vacuum is applied and subsequently released, it is important that the first and second surfaces 31, 32 of the layers are made of materials , thickness and surface finish such that they result in both a high coefficient of friction and a low adhesion connection.

The layers are such that the coefficient of friction between the first surface 31 and the second surface 32 of two adjacent layers 30 is greater than that of the materials normally used for lubrication. The coefficient of friction between the surfaces is above 0.6. More preferably the coefficient of friction is greater than 1.

In order to measure the coefficient of friction a standardized procedure can be used, such as ASTM D 1894 "Standardized test method for static and kinetic friction coefficients of film and plastic sheets".

An essential requirement of the layers is that they have low adhesion properties. Low adhesion means that the force of tangential adhesion between the first and second surfaces 31, 32 of two adjacent layers 30 is such that the layers 30 slide in relation to each other when there is no normal force. In fact, the first and second surfaces of the layers have low adhesion properties, such that a normal force per unit area below 0.07 N / mm2 is required to separate them (i.e. a normal compression force of -0.07 N / mm2) and / or the energy per unit area required to separate them in the normal direction is below 6.7 J / m2.

This is an important feature to prevent the different layers from sticking together once vacuum is no longer applied and element 1 regains its flexible state.

It should be considered that the friction and tack properties of the layers are influenced not only by the layer material, but also by the thickness and surface finish (roughness Ra) of the layer. This is why, in order to characterize the interface between the first and second surfaces 31, 32 of two adjacent layers 30, the friction and tack tests are done in two flexible layers 30 to take into account the effect of the manufacturing processes.

The adhesion properties have been measured by means of an adapted probe tack test with a pre-charge equivalent to atmospheric pressure and a waiting time of 100s, which is the maximum time scale mentioned in the standard so that the adhesive properties are established.

A standardized method for measuring adhesive adhesion is described in ASTM D2979-01, "Standardized Test Method for Pressure Sensitive Adhesiveness of Adhesives Using an Inverted Probe Machine". In the present case, an adapted version of this method has been used. The standardized method had to be modified since the adhesion between non-adhesive layers is being measured; these layers have a lower adhesion strength, and in them the adhesion depends on the material, thickness and roughness of the layers. The layers can be cleaned with alcohol (or any other means) before measuring, as indicated in the standard. However, it is possible to take the measurements without previously cleaning the surface of the layers, in order not to alter the adhesion properties of the layers in their conditions of use.

What is important is that the adapted method be repeated several times, in order to have a statistically significant number of measurements, so that it is possible to ignore atypical values.

The measurement method is shown schematically in Figure 10, in which the force (F) is represented with respect to the displacement (5) between the layers. The method of adhesion measurement is as follows:

a-c) The surfaces of the layers are approaching at a constant speed, and at some point they come into contact with each other and compress until the compression force Fmax is reached.

c-f) The surfaces of the layers are separated at a constant speed of 5 mm / min. (The standard establishes a speed of 10 mm / s, but a speed of 5 mm / min has been used to provide greater accuracy).

More specifically:

a-b) The surfaces move towards each other at a constant speed, with no contact between them.

b-c) Once there is contact between the surfaces, a compression force develops by movement, until it reaches its peak value of Fmax. Then, the movement between the layers stops and the force is maintained at the constant level Fmax.

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c-d) The surfaces leave each other at a constant speed. Initially the compression force is eliminated, to a point where a bond strength develops between the layers until a maximum Fadh value is reached. This adhesion force has a direction opposite to the initial compression force. d-e) The force of adhesion between the layers disappears through the disconnection of surfaces. e-f) The surfaces are separated from each other without contact, at a constant speed.

The specific adaptations to the standardized test are:

- A 50 mm circular contact surface is used instead of the standard 5 mm probe, because the adhesion forces are lower. In the case of the realization comprising tapes, a certain number of tapes must be placed next to each other, so that when aligned together they cover the circular surfaces of 50 mm.

- A normal vertical probe machine is used instead of an inverted probe machine, due to the lower adhesive forces.

- The adhesion test is carried out between the actual layers of the element, instead of performing the test between a stainless steel probe and the adhesive.

- The constant speed approach movement stops when the Fmax value is reached; At this point, the probe machine is programmed to keep the static load constant at Fmax. The Fmax value used is 200 N, which corresponds to a compression load of the order of magnitude of the atmospheric pressure on the surface.

- The probe rises vertically upwards from a resting surface during stages c-f.

- The static load is maintained for 100s (instead of 1 s).

- Actual layers are used in the test, instead of the thickness of the surface layer of adhesive specified in the standard.

- The values are averaged with at least five measurements of the same sample, without cleaning the surface between measurements.

- Adhesion is characterized by:

- Fadh / Surface, where Fadh is the maximum force measured while the surfaces are disconnected and Surface is the area of the contact surface, and

- By Wadh / Surface, where Wadh is the energy required to disconnect the two layers.

The following Table 1 shows examples of the layers, whose adhesion properties have been measured with the test

of tack by adapted probe just explained.

Table 1

 Layer specification (material / s, layer thickness)

 Maximum tack tension (N / mm2) Sticky Energy (J / m2)

 TPU Epurex® 4201 AU reinforced with fiberglass fabric (204 g / m2), 250 jm

 0.0259 2.2669

 TPU Epurex® 4201 AU, 75 um (in conditions of use)

 0.0424 0.8481

 BASF® 890 A10 TPU reinforced with fiberglass fabric (125 g / m2), 200 jm

 0.0222 1.2159

 BASF® S 85 A11 TPU reinforced with fiberglass fabric (125 g / m2), 200 jm

 0.0565 6.8287

A preferred embodiment of element 1 of the invention, example I, having the flexible layers 30 of Figure 2, includes thirty-seven layers 30, each layer having a thickness of 80 | jm. Each layer is made of thermoplastic polyurethane, in this case, from Epurex® 4201 AU (supplied by Epurex Films GmbH). The resulting element allows switching between a rigid state with a Young's module of 167 MPa (obtained at a negative pressure of -0.86 bar) and a flexible state with a Young's module of 22 MPa (measured at atmospheric pressure). Young's module has been obtained for a deformation of 0.2% -0.4%.

For certain applications, it is necessary that the element of the invention have a certain degree of rigidity. In order to obtain such stiffness the layers of the element can be improved, as shown in any of Figures 3-7.

Figure 3 shows another possible embodiment of layer 30a. In this case, the layer 30a comprises a core 40 coated by a first coating 41 on one side and a second coating 42, these first and second coatings constituting the first and second surfaces of the layer. Generally, the first and second coatings 41.42 are glued to the respective sides of the core 40.

Core 40 is essentially a continuous sheet of a flexible material that has a high Young's modulus. Having a high Young's module means that Young's core module is larger than Young's module of the materials used due to their elasticity (eg rubbers). In addition, Young's module of

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material constituting the core 40 is larger than Young's modulus of the coating material 41.42. The material that forms core 40 preferably has a Young's modulus greater than 0.2 GPa, such as low density polyethylene, LDPE, which gives the element the stiffness valid for certain applications.

Preferably, Young's modulus of core 40 material is greater than 0.8 GPa.

The core can be made of any of the following materials:

- Thermoplastics such as ABS, PEEK, PP, PEHD or PVC.

- Metals such as aluminum, brass or iron.

- Wood, paper.

It is possible that the first and second coatings 41, 42 on each side of the core 40 are made of the same material, including a specific layer thickness and surface finish, resulting in high friction coefficient and low adhesion properties, such as case of the layer shown in Figure 3. It is also possible that they are manufactured from different materials, including a specific layer thickness and surface finish, so that, when in mutual contact, each of the layers has the corresponding high friction and low adhesion properties.

Figure 4 shows another possible layer 30b for the element of the invention. In this case, the layer 30b comprises a core 40 and a first coating 41 only on one side of the core 40. The core 40 is made of a material having a high Young's modulus, and the coating 41 is made of a material with a minor Young's module. The thickness of the core 40 is greater than the thickness of the first coating 41. Also, in order to fully achieve the low adhesion and high friction properties between the layers 30b in this embodiment, the coating 41 has a smooth surface finish, while that the surface finish of core 40 is rough.

Additionally, in the embodiment of the layer 30a shown in Figure 3, it is important that the tangential bonding force between the layers 30a, that is, the tangential bonding force between the coatings 41.42 of two adjacent layers (which they are the surfaces in contact), be less than the maximum tangential bonding force due to the gluing between the coatings 41, 42 and the core 40 in each layer 30a. This is also an important feature, since otherwise the delamination of the layers can occur during flexion.

Similarly, in the embodiment of layer 30b of Figure 4, it is important that the force of tangential adhesion between layers 30b, that is, the force of tangential adhesion between the core 40 of a layer 30b and the coating 41 of the adjacent layer 30b (which are the surfaces in contact), is less than the maximum tangential bonding force due to the gluing between the coating 41 and the core 40 in each layer 30b.

Suitable materials for the coatings are: some thermoplastic polyurethanes, Acronal / Styrofan resin (40% Acronal® 12 with 60% Styrofan® D422, BASF), polyurea resin, silicone, rubber, silicone rubber, latex.

To further improve the stiffness properties of element 1 for those applications that require it (such as construction elements), the layers may comprise a fiber reinforced matrix.

As shown in Figure 5, layer 30c comprises a plurality of fibers 301 embedded in a matrix 302.

Figure 6 shows another layer 30d. This layer 30d is similar to the layers 30c shown in Figure 5. The difference is that the matrix 302 in the layer 30d has two portions 303 that do not have reinforcing fibers; These two portions 303 are only made of the matrix material.

The matrix material in the case of Figures 5 and 6 has the corresponding high friction and low adhesion properties.

A preferred embodiment of element 1 of the invention, example II, having the flexible layers 30c or 30d of Figures 5 or 6 includes six layers, each layer having a thickness of 250 µm. Each layer is made of thermoplastic polyurethane (from Epurex® 4201 AU) reinforced with 204 g / mm2 fiberglass (FG) fabric. The proportion of fibers is 73%. The resulting element allows switching between a rigid state with a Young module of 2876 MPa (obtained at a negative pressure of -0.86 bar) and a flexible state with a Young module of 84 MPa (measured at atmospheric pressure). Young's module was obtained for a deformation of 0.2% -0.4%.

Figure 7 shows another possible additional embodiment of the layers that constitute the laminar structure of the element.

The layer 30e in Figure 7 is similar to that of Figures 5 or 6, but in this case the matrix 302 reinforced with a plurality of non-woven unidirectional fibers 301 forms a core, which is further coated by the coatings 41,42 both Made of the same material with high friction and low adhesion.

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The fibers in the layers of Figures 5-7 are unidirectional nonwoven fibers. But it is also possible that the fibers are multidirectional or that the fibers are part of a woven fabric.

The fibers 301 in the embodiments shown in Figures 5-7 can be any of the following: glass fibers, carbon fibers, aramid fibers or polyester fibers. The matrix material 302 may be a thermosetting polymer, such as epoxy, polyester, polyuria, vinyl, phenolic, polyimide, polyamide, or a thermoplastic polymer, such as ABS, PP, PEHD, PEEK, PVC , PU, etc.

In the following examples, the coefficient of friction between surfaces made of a PUR resin (polyol) with a PUR hardener (isocyanate) with a 60 x 54 mm metal block sled, weighing 268 g, and a weight 0.2 N preload. The rest of the conditions used during the test of the materials are those described in ASTM 1894:

* Polyurea (RAKU-TOOL® PC-3411 resin with RAKU-TOOL® PH-3911 Isocyanate from RAMPF®), reinforced with bidirectional carbon fiber fabric (200 g / m2). Its coefficient of friction measured was 2.25.

* Polyurea (RAKU-TOOL® PC-3411 resin with RAKU-TOOL® PH-3911 Isocyanate from RAMPF®), reinforced with bidirectional fiberglass fabric (204 g / m2). Its coefficient of friction measured was 2.20.

These two examples are used to measure the coefficient of friction of the polyureas in the layer 30c in which the surfaces are made of the same material as the matrix 302 reinforced with fibers 301 (Figure 5).

When element 1 is used as an orthopedic device, it is able to adapt to the individual shape of the patient's limb. In its flexible state, element 1 adapts to the shape of the limb and when applied empty to element 1 is locked in its rigid state to provide support and stabilization. For this purpose, it is important to have a high stiffness ratio between the flexible and rigid states, in addition to each layer being preferably made of a material with a high Young's modulus. In an ideal case, the layers 30, when in the rigid state, are completely glued to each other through the negative pressure applied, the stiffness of the element is n2 times greater than in the flexible state under atmospheric conditions, the number being n of layers in the element. In the real case, this increase in stiffness factor of n2 approximates, depending on the actual coefficient of friction that can still allow some slippage between the layers. For example, an element that is 2.4 mm thick, made of 6 layers of 0.4 mm each, has a stiffness relationship between the rigid and flexible states of 36 (= 62). The stiffness ratio to be achieved by the element depends on the type of application. For example, a proportion of 4 is not sufficient in the case of orthotic adjustment. For orthotic adjustment, an element with twelve thin layers, and therefore a ratio of 144, works. But it is also possible to double the thickness of the layers, include only six layers and the resulting element is flexible enough and is capable of achieving a similar stiffness suitable for orthotic applications.

In order to apply a homogeneous force during the compression of the different layers, as shown in Figure 8, the element 10 further comprises an air permeable layer 50, such as a crosslinked structure, inserted in parallel with the layers 30a in the inside of the flexible envelope 10. The air permeable layer 50 allows the vacuum to be distributed evenly. For example, a plastic mesh made with 100 µm diameter fiber and open cells of approximately 3 x 3 mm, provides a uniform pressure distribution.

The valve 20 is inserted into the envelope 10 on the side next to the air permeable layer 50. This prevents blockage of the air flow by a layer 30a that sticks to the valve hole. Additionally, the air-permeable layer 50 prevents the outer layer from adhering to the envelope, which could lead to loss of flexibility in the flexible state.

As indicated above, it is desirable that the layers be manufactured from a material with a high Young's modulus to manufacture an element with a high stiffness state, but materials that have a high Young's modulus are generally low extensibility. Because they are not extensible, they cannot be adapted to any 3D shape. In order to adapt to any conformation or 3D shape, especially those that have irregular surfaces, in another possible embodiment of the invention (as shown in Figure 9), the layer 30f is provided in the form of woven tapes or strips for add degrees of freedom to the layer. To keep this structure organized after repeated use and avoid overlapping and losing the tapes, the edges of any 2D pattern can be sewn and cut, taking care to ensure that both ends of each tape have been sewn into the pattern.

In order to manufacture the tapes, the warp and weft tapes 60, which form the layer 30f, any of the flexible layers 30, 30a, 30b, 30c, 30d or 30e of the above embodiments is cut into tapes or strips of desired width, which are woven later.

In cases where the layer is a composite material made of a fiber reinforced polymer matrix, such as layers 30c, 30d or 30e, of Figures 5-7, it is also possible to directly fabricate the composite material with the width specifies

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Making the smallest fabric, is dedr, with ribbons of smaller width, allows a better fit. For the adjustment of the human body, the 3 x 1 twill fabric made with a tape or strip with a width of 4 mm and with a separation of 1 mm between the ribbons / strips provides a good result. But any tape width can be used depending on the purpose.

The tapes or strips can be manufactured with a 600tex fiberglass wick (PPG) flattened with a width of 4 mm. The fiber wicks are then impregnated with thermoplastic polyurethane (such as Elastollan® 890 A10 from BASF), respecting a matrix / fiber volume ratio of approximately 30/70. In this case, the surface roughness must be approximately 1.27 to achieve the correct friction and tack properties of the first and second surfaces.

It is also possible to manufacture a smaller fabric using a 300tex fiberglass wick (PPG) flattened to a width of 2.5 mm.

A preferred embodiment of element 1 of the invention, example III, which has the flexible layers 30f shown in Figure 9, includes six layers, each layer having a total thickness of 450 µm. Each layer is made of tapes or strips with a thickness of 160 jm, woven in a 3 x 1 twill fabric. The tapes are made with a thermoplastic polyurethane matrix of Epurex® 4201 AU reinforced with fiberglass wicks (FG) of 600tex that are embedded in a matrix. The fiber ratio, in this case, is 60%. The resulting element allows switching between a rigid state with a Young's modulus of 546 MPa (obtained at a negative pressure of -0.86 bar) and a flexible state with a Young's modulus of 24 MPa (measured at atmospheric pressure). Young's module was obtained for a deformation of 0.2% -0.4%.

As for the fabrics, many solutions are possible: normal, five-point hedge, 16-point hedge, 2 x 2 twill or 3 x 1 twill.

Twill fabric has the advantage of being more flexible and draping more than normal tissue. The weave fabric is also a good option in terms of fall capacity.

In any case, regardless of the fabric used (normal, twill, hedge, ...) in the preferred embodiment there is a gap, approximately 1 mm, between each tape in both the warp and weft directions, in order to allow a certain degree of freedom between separate tapes and thus obtain sufficient fall capacity to fit the human body.

The following Table 2 summarizes the main characteristics of the three examples I, II, III provided above for the layers inside the element and their properties. In all three examples, the layers of the element are enclosed in a sealed PP / HDPE bag. The element also contains a nylon mesh for the distribution of the vacuum, adding a total of 0.6 mm in the thickness of the element.

Table 2

 Ex

 Fiber Ratio Specification (%) Coef. Static Friction Tension of Stickiness (MPa) (N / mm2) Energy of Stickiness (J / m2) Young's Module. - rigid (MPa) (at -0.86 bar) Young-flexible module (MPa) (at atm pressure)

 I

 37 layers of Epurex® 4201 AU TPU Fiber-free 1.41 -0.04 0.85 167 0.12

 II

 6 layers of Epurex® 4201 AU TPU reinforced with FG fabric (204 g / m2) 73% FG 7.59 -0.03 2.27 2876 84

 III

 6 layers twill 3x1 woven tape Epurex® 4201 AU TPU tapes reinforced with FG U600tex * 60% FG 4.50 -0.03 1.65 546 24

In this text, the term "comprises" and its derivations (such as "understanding / comprising", etc.) should not be understood in an exclusive sense, that is, these terms should not be construed as excluding the possibility that what is describe and define may include elements, stages, etc. additional.

In the context of the present invention, the term "approximately" and the terms of your family (such as "approximate", etc.) should be understood as indicating values very close to those that accompany the term mentioned above. That is, a deviation within reasonable limits with respect to an exact value

It should be accepted, as one skilled in the art will understand that such deviation from the indicated values is inevitable due to measurement inaccuracies, etc. The same applies to the terms "over" and "around" and "substantially."

Claims (15)

5 10 fifteen twenty 25 30 35 40 Four. Five fifty 55 60 1. Element (1) with variable stiffness controlled by negative pressure, the element comprising: - a hermetic wrap (10); - a plurality of flexible layers (30, 30a, 30b, 30c, 30d, 30e) in the envelope, each layer (30, 30a, 30b, 30c 30d, 30e) having a first surface (31,41) and a second surface (32, 42); Y, - a valve (20) adapted to evacuate the inside of the envelope (10); characterized by: - the first and second surfaces (31, 41, 32, 42) of two adjacent layers have a coefficient of friction between them that is greater than 0.5; - the first and second surfaces (31,41, 32, 42) of two adjacent layers have adhesion properties such that a normal force per unit area below 0.07 N / mm2 is necessary to separate them and / or the Energy per unit area required to separate them in the normal direction is less than 6.7 J / m2. 2. Element (1) according to claim 1, wherein the layers (30, 30f) are made of a single material. 3. Element (1) according to claim 1, wherein the layers (30c, 30d, 30e, 30f) are made of a plurality of fibers (301) immersed in a matrix (302). 4. Element (1) according to claim 3, wherein the fibers (301) are unidirectional. 5. Element (1) according to claim 3, wherein the fibers (301) are bidirectional or multidirectional. 6. Element (1) according to any of claims 3-5, wherein the fibers (301) are selected from fiberglass, carbon, aramid or polyester fibers. 7. Element (1) according to any of claims 3-6, wherein the matrix (302) is made of a thermosetting polymer or a thermoplastic polymer. Element (1) according to any one of claims 1-7, wherein each layer (30a, 30b, 30d) further comprises a core (40) coated by at least one coating (41,42) on one side of the core (40). 9. Element (1) according to any of claims 1-7, wherein each layer (30a, 30d) further comprises a core (40) coated with respective first and second coatings (41, 42), a coating to each core side (40). 10. Element (1) according to any of claims 8-9, wherein the coatings (41,42) are made of a thermoplastic polyurethane elastomer. 11. Element (1) according to any of claims 1-10, wherein the layers (30a, 30b, 30c, 30d, 30e) are made of a continuous sheet. 12. Element (1) according to any of claims 1-4 and 6-10 when dependent on claim 4, wherein the layer (30f) is made of a woven structure of tapes (60, 65). 13. Element (1) according to any preceding claim, in which the adhesion properties of the layers are measured using an adapted probe tack test with a pre-charge equivalent to atmospheric pressure and a pre-charge wait time of 100s. 14. Element (1) according to claim 12, wherein the adhesion properties of the layers are measured using an adapted probe tack test with a pre-charge equivalent to atmospheric pressure and a pre-charge wait time of 100s, making such measurements on a certain number of tapes (60, 65) placed adjacent to each other on a flat surface. 15. Element (1) according to any preceding claim, in which the adhesion properties of the layers are measured on layers in their conditions of use.